Great questions! Some people prefer the look of the over-the-mirror vanity light (and some bathrooms may not be large enough to accompany much else or you may be living in a house you can't renovate to that degree). But using a sconce on each side of the mirror is also a great idea and helps to provide an even wash of light at face level, where you'll need it most when you are visiting the bathroom mirror.

While you do need to be mindful of wattage and make sure you don't overload the fixture with the bulbs you choose, there are other measurements of light that you need to know, including lumens and color temperature.

Lumens are the raw quantity of light given by a light source. There are some tables about lumens and watts over here so you can easily convert between the two measurements, which you will see together on a box of bulbs: http://www.lightsonline.com/how-to-choose-incandescent-light-bulbs

Color temperature refers to how white the light looks and is measured in degrees Kelvin. The higher the temperature, the whiter the light, so the soft yellow-y glow often associated with incandescent bulbs has a low color temperature while pure daylight would have a very high one. That, too, is on a box of bulbs as a sliding scale.

Here is more information about lighting the bathroom: http://www.lightsonline.com/bathroom-lighting-ideas

Lights Online is very correct. Fixture installed over your head will produce shadows below our cheekbones. That's why make-up mirrors in theater dressing rooms have light sources all around the mirror. Here is a simple bathroom with good fixtures at the proper location and height -

The size of the mirror and bathroom need to be considered as well as the color of the walls and counter top. Most of the time just sconces on the sides will not provide enough light for a mirror used to groom every day. Other lights in the room or additional recessed cans above the vanity are usually necessary if your sconces don't total about 3000 lumens.

Being well aware of the side lighting vs. bar lighting for vanities debate, I simply prefer the bar lighting, and I don't have the spooky shadows in my bathroom that everyone says will happen with bar lights. That may be because I have other sources of light besides the bar above the vanity. Therefore, I agree with Fred that you should have other sources of lights in the room: a ceiling fixture, recessed cans, windows or skylights, etc. And don't blind yourself with too much vanity lighting, be mindful of the wattage totals (or lumens).